Isolated knock/stumble after sitting a while (603) ...

[BodhiBenz1987]

I had my 87 300D sitting in the garage since late October while I did a bunch of bodywork (that will be a thread in the restoration section later!) ... finally finished today, thrilled to take it out. It started up super easily, ran nice and smooth for about 3 minutes as I let it idle while I closed up the garage. All of a sudden it just stumbled and made two horrendous knocks, kicked out a little black smoke, and then went back to normal. Nothing remotely off since. It drives great, great power, smooth idle, no unusual smoke. Was this maybe a fuel issue from sitting over the winter? I've never had anything like that before, other than some stumbles right after starting in the deep cold. But it was 70 today, and the car ran fine for a few minutes before it happened. Pretty pronounced knocks ... thought I'd have a heart attack, but then it just smoothed out and no problems. Should I worry about this or does it sound like something that could have been a sitting issue? I'm assuming if it were something terrible, the car wouldn't proceed to go back to running wonderfully.
Other than that, great to have the car back on the road just as the weather gets nice. It's such a pleasure to drive.